The present disclosure relates to an illuminating apparatus used to illuminate hard-copy documents for digital recording, such as in digital scanners, facsimile machines, and digital copiers. In particular, it relates to an illuminator which includes a phosphor material interposed between a light source and the hard-copy document, which provides a more even illumination.
In office equipment, such as digital copiers and facsimile machines, original hard-copy documents are recorded as digital data using what can be generally called a “scanner.” In a typical scanner, a document sheet is illuminated and the light reflected from the document sheet is recorded by a photosensitive device such as a CCD (charge coupled device) or CMOS (complementary metal oxide semiconductor) array (also known as a contact image sensor (CIS)), to be converted to digital image data. Successive narrow strips of the document sheet are illuminated as the sheet is moved through a document handler, or the photosensitive device is moved relative to a platen on which the document sheet is placed. These narrow strips of document image are then assembled by software into a complete image representation of the original document.
CIS scan bars used for document scanning have used a variety of illumination sources, including light emitting diodes (LEDs) and cold cathode fluorescent lamps (CCFL). CCFL illumination can be used directly for monochrome scanning, or by using sensors with an array of RGB (Red, Green, and Blue) color filters over the pixels, CCFL white illumination can be used to scan color images. While CCFL illumination tends to be very uniform and provides white light, such lamps generally employ high voltages and have a high power consumption. LED-based illuminators typically employ a light-transmissive element that exploits internal reflections to direct light from LEDs to emerge in substantially parallel rays from an exit surface of the element toward a document. Scan bars of this type often use a single LED for monochrome scans or a red, blue, and green triplet of LEDs (e.g., based on InGaAlP, InGaN, and GaP) to capture color images. For color scans, the RGB LEDs are turned on one at a time in succession, in order to capture three separate images, one illuminated with each color, from which a full color scan image is then assembled. The illuminator includes a prism for spreading the illumination from each of the three LEDs (RGB) across a strip of the document as uniformly as possible.
Designing an illuminator for a scanner presents challenges in providing, among other aspects, an even illumination along the narrow strip of the document, i.e., in the fast scan direction. Some of these LED-based illuminators use a lenticular, notched rear surface, on the side of the illuminator prism furthest from the document target, which catches the light rays traveling down the length of the illuminator and reflects these rays in a direction which is approximately perpendicular to the longest axis of the scan bar. The reflected rays then exit the illuminator prism from a front surface and illuminate the target document surface. Other scan bars use a white paint pattern on the rear surface, which modifies the index of refraction, as compared to an optical surface exposed to air, to accomplish a similar effect. The notches or paint patterns have a pattern which varies down the length of the light pipe prism in an effort to balance the illumination at the near-end of the prism (closest to the LEDs), where the illumination would otherwise tend to be brightest, with the illumination at the far-end of the prism (farthest from the LEDs). For example, in the notched designs, the notches are deeper and larger at the far-end and shallower at the near-end. In this design, the smaller notches at the near-end where the illumination would be brightest redirect less of the total illumination in the direction of the document, and the larger notches at the far-end are intended to catch more of the total illumination to compensate for being farther from the LED source. However, such LED illuminator light-pipe prisms still exhibit significant illumination non-uniformity. Specifically, they tend not to provide uniform illumination down the length of the scan bar, or when comparing one color to another. Secondary reflections from the far-end of the prism and other scattered light rays tend to make precisely uniform illumination difficult to achieve. Additionally, this design tends to be non-uniform between the specular reflective, and diffuse illumination domains. This last type of non-uniformity is particularly problematic because any correction applied to reduce non-uniformity in one domain tends to result in an increase in non-uniformity in the other. It is generally not possible to calibrate differently for the two domains at one time, and some document surfaces are a combination of specular and diffuse illumination surfaces (e.g., a shiny pebble-grained surface where portions of the shiny surface reflect in the specular domain with the rest being illuminated in the diffuse domain.
Irregularities in the illumination level in the illuminated area can result in defects in the image data, which may not be completely correctable in software, particularly in the case of discrete light sources, such as LEDs.